Rosetta Sees Signs of Water Ice on Comet Surface (Photos)

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Patches of water ice appear to be speckled across the surface of
a comet, according to a new study using observations from a
European space probe.

The
Rosetta spacecraft, currently orbiting Comet
67P/Churyumov–Gerasimenko, spotted 120 bright, reflective spots
on the surface of the comet that were at least a few meters
(about 6 feet) in size. While their composition is still being
examined, the spots tend to appear in areas that are shaded by
the sun, scientists noted. The researchers also note that there
have been no significant changes to the spots after a month of
observations.

"At the time of our observations, the comet was far enough from
the sun such that the rate at which water ice would sublimate
would have been less than 1 mm per hour of incident solar
energy,"
said Pommerol, who is lead author on a study analyzing the
bright spots. "By contrast, if carbon dioxide or carbon monoxide
ice had been exposed, it would have rapidly sublimated when
illuminated by the same amount of sunlight. Thus, we would not
expect to see that type of ice stable on the surface at this
time."

The spots are up to 10 times brighter than the average surface
brightness of the
comet, as measured by Rosetta. Sometimes they appear
together, particularly when they are at the bottom of cliffs. The
research team speculates this is because the cliff wall recently
eroded or collapsed, revealing material below the dusty surface.

In other locations, bright spots have been observed alone.
Scientists believe these instances represent objects that lifted
off the surface when the comet's motion was more active. These
objects, however, were not moving fast enough to escape the
comet's gravitational pull altogether.

Comets are known to be rich in ices, based on observations of the
trails they leave behind. When comets approach
the sun, the heat of the star warms the comet's surface and
causes the ice to melt. This process also releases dust clumped
near the ice. But not all of the dust escapes, leaving the
surfaces of comets very dark as the dust settles back on to the
surface, or fails to leave in the first place.

It's not clear when the ice patches formed, but the team has two
hypotheses. The first suggests that when 67P was closest to the
sun, 6.5 years ago, cometary activity pushed the icy chunks into
shadowed regions and protected them from the sun. Alternatively,
perhaps carbon monoxide and carbon dioxide beneath the surface
pushed the ice around while the comet was farther from the sun.

The team also did some tests in the lab to see what happens when
water ice is combined with other minerals. When this ice is
exposed to simulated solar illumination, a dust layer of a few
millimeters thick is created, the researchers discovered. This
thin layer could be enough to obscure the ice below on comets.
But if a bigger chunk of dust is pushed aside by cometary
activity, it would show off the ice below.

Results from the observations were published in the journal
Astronomy & Astrophysics and are based on observations by the
OSIRIS narrow-angle camera taken last September, one month after
Rosetta arrived at the comet.